A polyvinylidene fluoride resin (hereinafter abbreviated as “PVDF resin”) is a crystalline resin having a low glass transition temperature and is excellent in heat resistance, chemical resistance, mechanical properties (for example, tensile strength, flexural modulus, flexural strength, compressive strength and impact resistance), abrasion resistance, flame retardancy and weatherability. The PVDF resin also develops extremely specific electrical properties in cooperation with a feature that a dipole moment of a C—F bond in its molecular structure is high.
Other basic properties of the PVDF resin include good melt processability. In other words, the PVDF resin has a wide processable temperature range from its melting point to its decomposition point and exhibits good melt-flow characteristics.
As described above, the PVDF resin has well balanced physical properties and good processability in combination, and application fields thereof extend to a wide variety of fields of abrasion-resistant materials, weatherable materials, electric and electronic materials, leisure materials, and the like.
The PVDF resin is excellent in the suitability for secondary processing such as machining, bending or welding after primary processing. For example, a machining stock such as a thick-wall round bar or plate is produced by extrusion molding making use of the PVDF resin. The machining stock is machined into a desired shape by milling, perforating, cutting, combinations thereof, or the like.
However, the PVDF resin has a drawback that a molten resin is colored when a molding temperature becomes high in melt molding such as injection molding or extrusion molding. In order to avoid the coloring of the molten resin, the molding temperature in the injection molding or extrusion molding is generally controlled in such a manner that the temperature of the molten resin is 280° C. or lower.
In the injection molding or extrusion molding, pellets of the PVDF resin are generally used. For example, when a molding such as a wafer carrier, joint or valve is molded by injection molding, a screw injection molding machine is ordinarily used. The pellets are fed into a heated cylinder of the injection molding machine by rotation of a screw. The pellets are fed by the screw and at the same time, evenly melted. The molten resin is injected into a mold from the injection molding machine.
In the extrusion molding, the pellets are fed into a heated cylinder of an extrusion molding machine by rotation of a screw, and the molten resin is extruded into a shape of, for example, a rod, plate or pipe from a forming die installed in the tip of an extruder.
In such a conventional melt molding process, the PVDF resin is subjected to thermal history upon the formation of pellets and further subjected to thermal history even upon the melt molding, so that it is difficult to prevent the coloring of the resulting molding by only controlling the molding temperature upon the melt molding low. An extrusion-molded product used as a machining stock, to say nothing of an injection-molded product, is required to have a beautiful color tone and be little colored. In recent years, there has been a strong demand for provision of a PVDF resin molding having high performance and high quality, and so it is required to develop a new technique for inhibiting coloring upon melt molding.
More specifically, PVDF resins can be synthesized by various polymerization processes. In industrial production, however, the resins are synthesized by an emulsion polymerization process and a suspension polymerization process. A PVDF resin synthesized is collected in the form of resin powder. Powder characteristics of the resin powder such as average particle diameter, particle size distribution and evenness of particle shape vary according to the kind and amount of a polymerization initiator, the kind and amount of a suspending agent or emulsifier, the kind of a reaction medium, a polymerization temperature and a manner of combining them. In the emulsion polymerization process, a latex having a small particle diameter of about 0.2 to 0.5 μm is formed and is subjected to a granulation treatment using a flocculant after the polymerization.
When it has been intended to produce moldings with PVDF resin powder by melt molding such as injection molding or extrusion molding, it has been impossible or extremely difficult to obtain moldings having a fixed shape and quality because the powder has been unable to be stably fed into a cylinder of a molding machine, or an extrusion rate has become unstable.
Therefore, the PVDF resin powder has heretofore been not fed to injection molding or extrusion molding as it is in a state of resin powder, but fed after melt processing into pellets (for example, U.S. Pat. No. 6,846,436 B1). A pelletizing step has been considered to be necessary even for evenly kneading the PVDF resin powder with various kinds of additives.
When the PVDF resin powder is pelletized, the PVDF resin is subjected to thermal history in addition to the fact that it takes time, and the cost becomes high. Therefore, when the resultant pellets are used to conduct melt molding, the pellets are subjected again to thermal history at a high temperature, so that such pellets have involved a problem that the resultant molding is colored even when the molding temperature is controlled low.
However, no PVDF resin powder capable of being stably melt-molded has heretofore been proposed. A PVDF resin for melt molding is marketed in the form of pellets. Some PVDF resins are marketed in the form of resin powder. It has however been general that when injection molding or extrusion molding is conducted, the resin powder is pelletized and then fed into a molding machine.
Since the pellets are not a final molding, there is no need to strictly control a scatter of shape. Accordingly, a single-screw or twin-screw extruder is ordinarily used in pelletizing of the PVDF resin. However, the processing conditions may not be strictly controlled because the resin is melt-extruded into a fine strand from a die nozzle and cut into a proper size, or a resin ejected is only cut on the tip of an extruder unlike general melt molding into a molding.
On the other hand, in injection molding or extrusion molding of the PVDF resin, it is necessary to use a molding machine suitable for it, stably feed a resin material to the molding machine and strictly control an extrusion rate and molding conditions so as to mold a molding having a fixed quality.